Can we access GMAIL API using Service Account?

Question

I have a desktop application to read mail using GMAIL API over REST Interface. I want to use service account so that we can download the mails using domain setting and user interaction is null. I am successfully able to create Gmail Service instance but when I try to access any Gmail API method like fetching mail list or any other I get an exception saying

Google.Apis.Auth.OAuth2.Responses.TokenResponseException: Error:"access_denied", Description:"Requested client not authorized."

I am done with all the setting at developer console and added scopes to my gapps domain.

Does Gmail API support service account? Using the same setting and service account I am able to get list of all files in Google drive using Drive service and API.

Answer 1

I use the following C# code for accessing Gmail from Service Account

String serviceAccountEmail =
    "999999999-9nqenknknknpmdvif7onn2kvusnqct2c@developer.gserviceaccount.com";

var certificate = new X509Certificate2(
    AppDomain.CurrentDomain.BaseDirectory +
        "certs//fe433c710f4980a8cc3dda83e54cf7c3bb242a46-privatekey.p12",
    "notasecret",
    X509KeyStorageFlags.MachineKeySet | X509KeyStorageFlags.Exportable);

string userEmail = "[email protected]";

ServiceAccountCredential credential = new ServiceAccountCredential(
    new ServiceAccountCredential.Initializer(serviceAccountEmail)
    {
        User = userEmail,
        Scopes = new[] { "https://mail.google.com/" }
    }.FromCertificate(certificate)
);

if (credential.RequestAccessTokenAsync(CancellationToken.None).Result)
{   
    GmailService gs = new GmailService(
        new Google.Apis.Services.BaseClientService.Initializer()
        {
            ApplicationName = "iLink",
            HttpClientInitializer = credential
        }
    );

    UsersResource.MessagesResource.GetRequest gr =
        gs.Users.Messages.Get(userEmail, msgId);
    gr.Format = UsersResource.MessagesResource.GetRequest.FormatEnum.Raw;
    Message m = gr.Execute();

    if (gr.Format == UsersResource.MessagesResource.GetRequest.FormatEnum.Raw)
    {
        byte[] decodedByte = FromBase64ForUrlString(m.Raw);
        string base64Encoded = Convert.ToString(decodedByte);
        MailMessage msg = new MailMessage();
        msg.LoadMessage(decodedByte);
    }
}

Answer 2

If you want to "read mail" you'll need the newer Gmail API (not the older admin settings API that 'lost in binary' pointed out). Yes you can do this with oauth2 and the newer Gmail API, you need to whitelist the developer in Cpanel and create a key you can sign your requests with--it take a little bit to setup: https://developers.google.com/accounts/docs/OAuth2ServiceAccount#formingclaimset